1. Field of the Invention
This invention relates to a substrate heating apparatus for forming thin films on a substrate surface, and more particularly to a substrate heating apparatus suitable for the use in a CVD (Chemical Vapor Deposition) technique.
2. Description of the Prior Art
Conventionally, in the process of manufacturing semiconductor devices, a CVD technique has been employed to form thin films of various kinds on a substrate. Specifically, material gas is supplied to the substrate so as to form thin films on the surface thereof by chemical reaction. In recent years, a low-pressure chemical vapor deposition (LPCVD) technique has been mainly employed, wherein thin films are formed on the substrate under a lower pressure in order to enhance the uniformity of film thickness and film quality. To accelerate such chemical reaction, and to form good quality thin films, the substrate is usually heated up to a desired temperature.
The substrate is usually heated by use of the following techniques. Specifically, the wafer is placed on the heater which is heated by resistance heating or high-frequency induction heating. Further, the substrate is directly heated by infrared ray-lamp heating.
FIG. 12 is a schematic cross-sectional view illustrating a conventional low-pressure CVD apparatus into which a resistance heater is incorporated. In FIG. 12, a container 101 is made of aluminum alloy, and the wall thereof is water-cooled by use of a water cooling pipe 102. The container 101 has a reaction chamber 103 in which thin films are formed on a substrate 107 by use of the low-pressure CVD technique. Material gases are introduced into the reaction chamber 103 through a material gas supply pipe 104. The pressure of the reaction chamber 103 is reduced by use of an exhaust system (not shown) through a vent 105 provided in the bottom of the reaction chamber 103. A quartz susceptor 106 is disposed at the bottom of the reaction chamber 103. A heater 108 which heats the substrate 107 is provided on the quartz susceptor 106. The quartz susceptor 106 serves as a thermal insulating member that prevents the heat of the heater 108 from radiating outside. The heater 108 comprises a tungsten wire 109 covered with an aluminum oxide 110. The tungsten wire 109 is connected through copper leads 111 to a power source (not shown). The tungsten wire 109 is energized to generate heat such that the substrate 107 is heated to hold a prescribed constant temperature. The quartz susceptor 106 and the heater 108 constitute a substrate table.
FIG. 11 is a graph illstrating the relationship between the pressure in the reaction chamber 103 and the respective surface temperatures of the heater 108 and the substrate 107. Specifically, I represents the surface temperature of the heater, and II represents the surface temperature of the substrate. FIG. 11 shows the change of the surface temperature II of the substrate with respect to the change of the pressure in the reaction chamber while the surface temperature I of the heater is being held constant.
As can be seen from FIG. 11, the difference between the surface temperatures I and II is held small under a pressure substantially the same as the atmospheric pressure. This is because heat generated by the heater is carried to the substrate both by radiation and by the convection of vapor between the heater and the substrate under such a pressure. However, there is substantially no convection of the vapor under a lower pressure. Thus, the heat generated by the heater is carried to the substrate only by radiation, resulting in a lower efficiency of heat conduction. Therefore, in order to raise the surface temperature of the substrate to a desired temperature, the temperature of the heater must be much higher than the desired surface temperature of the substrate.
When thin films are formed on the substrate 107 under the lower pressure, an undesirable thin film 112 is inevitably formed on the surface of the heater 108, as shown in FIG. 13. This is because the surface temperature of the heater 108 is higher than that of the substrate 107. If such thin film formations are repeatedly performed, the thickness of the undesirable thin film 112 increases and eventually peels off the heater 108 resulting in dust. Further, the material gas is consumed in the vicinity of the surface of the heater 108. Thus, the growth rate of the desirable thin films on the substrate 107 is decreased.
To solve the above-described disadvantages, the inventor of the present invention has disclosed an apparatus wherein the peripheral portion of a substrate is covered with a quartz cover member (see Japanese Patent Laid-open Publication No. 63-18079). However, this quartz cover member alone is not sufficient to suppress the undersirable heat conduction toward the peripheral portions of the substrate. Specifically, when this apparatus is used continuously for a long time, the undesirable thin films is deposited on the quartz cover member by consuming the material gas. Thus, further improvement thereof has been desired.